This invention relates to a control apparatus for use in an A.C. elevator in which an induction motor for running a cage is driven by a variable-frequency power source.
An A.C. elevator uses an induction electric motor for driving the cage of the elevator, and the induction motor is supplied with the output of a variable-frequency power source, whereby a torque control is performed by varying a slip frequency. In this regard, there has been proposed a method in which the frequency and voltage of the power source to be applied to the induction motor are controlled so as to prevent regenerative power from developing in the induction motor in case of a braking mode or an unloaded operation (a heavy load descent operation or a light load ascent operation).
FIG. 2 is a simplified equivalent circuit diagram of an induction motor for explaining the above method of preventing the development of regenerative power. In the figure, symbols l.sub.1 and l.sub.2 denote leakage inductances on the primary side and secondary side of the induction motor respectively, and symbols r.sub.1 and r.sub.2 resistances on the primary side and secondary side respectively. Letter S indicates a slip, and letters V and I indicate a voltage applied to the induction motor and a current flowing therethrough, respectively.
Here, assuming the slip S to be: EQU S=-r.sub.2 /r.sub.1 ( 1)
a mechanical input P.sub.m becomes: ##EQU1## On the other hand, electric power P.sub.E which is consumed in the induction motor becomes: EQU P.sub.E =(r.sub.1 +r.sub.2)I.sup.2 ( 3)
so that the mechanical input and the power consumption in the induction motor equalize. Accordingly, when the induction motor is operated in the slip state satisfying Eq. (1), no regenerative power develops from the induction motor, and the supply of electric power is unnecessary. Meanwhile, letting .omega..sub.r denote the rotational angular velocity of a rotor and .omega..sub.O denote the input angular frequency, the induction motor generates a torque T given below: ##EQU2## Here, substituting Eq. (1) into Eq. (4), EQU T=(r.sub.1 .omega..sub.0)I.sup.2 ( 5)
Further, when S=-r.sub.2 /r.sub.1 indicated in Eq. (1) is applied to FIG. 2, the following relation between the voltage V and the current I is obtained: EQU V=(l.sub.1 +l.sub.2).omega..sub.0 I (6)
When this equation (6) is substituted into Eq. (5), the following is obtained: ##EQU3## Thus, the development of the regenerative power can be prevented by controlling the voltage V on the basis of a torque command.
With the above control method, however, the torque T is inversely proportional to the cube of the input angular frequency .omega..sub.0 as indicated in Eq. (7), and a controllability therefor is very inferior. Another problem is that, when the slip S is changed stepwise to the value indicated by Eq. (1) upon the shift of the torque command from power running to braking, current flows to the primary side to generate a transient torque on account of residual current or residual magnetism remaining in the rotor, so a comfortable ride in the cage is not achieved.